The goal of the proposed research is to study the etiology of drug use in a large sample of middle-aged, same sexed twins reared apart from early childhood and a matched control sample of twins reared together. The twins have been identified in the Finnish Twin Registry. A questionnaire concerning use of common drugs, such as tobacco, alcohol, tranquilizers and sleeping pills, will be constructed and sent to the twins. Analyses will focus on the estimation of hereditary as well as environmental influences on the use of these drugs. A major set of analyses will consider the etiology of polydrug use through estimates of "genetic correlations" and "environmental correlations." In addition, the design will permit analyses of: (1) the effect of age of separation of twins in smoking and non-smoking families in order to assess the influence of early "passive smoke" on later tobacco use; (2) the effect of age of separation of the twins in order to assess the influence of early rearing similarity on later similarity in drug use; (3) the interaction between the etiology of drug use and age; (4) the interaction between the etiology of drug use and gender; (5) the extent to which etiological factors involved in extreme drug use are similar to those involved in less extreme drug use; and (6) genotype-environment interaction.